EVERY animal tries to avoid unwelcome contact with others for which it adopts different methods. Some have colourful bodies, which cannot be easily distinguished from their surroundings, whereas some use colours, which are usually bright and conspicuous, as a warning to discourage others from approaching. Both the strategies are very effective and help animals in gaining advantage in their struggle for survival. The interesting part of the story is that these colours, or camouflage, are used by both the prey species and the predators. The difference is that the former use them to avoid predators, while the latter employ them to conceal themselves from the prey they wish to catch.

Smaller invertebrates, particularly insects and small water creatures such as shrimps are preyed upon by larger animals and need their colours to enable them to avoid capture as often as possible. The caterpillars of many moths closely resemble twigs of the trees on which they are to be found, and birds are unable to distinguish them. Other insects escape attention because they resemble objects that are not usually a source of food; swallowtail butterfly, caterpillars, for instance, resemble bird-droppings due to black and white saddle on their backs. Naturally, no bird would be interested in investigating its own droppings for food unless it is very hungry.

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Many predators also blend well with their natural surroundings so that they can stalk the prey unnoticed. The white coats of polar bears and other Arctic hunters, like arctic foxes and snowy owls, blend with the snow and ice so perfectly that it is not easy to spot them. Same way the stripes and spots of zebras, leopards, giraffes and tigers blend matchlessly well with the moving patters of light and dark among the grassy plains and scattered trees of the tropical savanna that they inhabit.

Most small animals, particularly insects that gain protection by assuming colours that match with their background, depend for their camouflage on their presence in very particular surroundings. But there are some that have overcome these limitations and adapted themselves in such a way that they can change their colours to match a new background, often quite perfectly, as do chameleons. These small, tree-living reptiles can be green, yellow or cream or dark brown, often with darker or lighter spots, to blend with different backgrounds. This helps to conceal them from their snakes and bird enemies.

Many sea creatures are also masters of disguise. At night the whole body of the shrimp becomes a beautiful translucent blue whatever the daytime colour may have been - green, brown or patterned. A crab known as Uca has a remarkably constant rhythm of colour change. It is pale at night and dark during the day and these changes are attuned to the tides. The organs that contain the coloured pigments are always at their largest and darkest just before low tide. This is the time when Uca is most actively foraging for food and so is also the time when it needs the most concealment.

Crabs in different localities have their own rhythms of colour change synchronised with the different time of low tide. The uca obviously has some kind of internal clock that maintains the rhythm of its colour changing, but like most biological clocks its mechanism is still a mystery.

Flounder, a fish, is another well-known animal that can change colour. It can put on a speckled appearance against an unevenly coloured background, or a uniform colouring against a plain background. Many other sea creatures are adept at changing colour, and one of the most versatile is the octopus that can change its colours and colour patterns to match practically any sort of background. This is possible because the pigments that give the animal its various colours are contained in many special organs called chromatophores, which vary in appearance according to their degree of expansion. The octopus has two sets of chromatophores, one varying from black to red-brown, and the other from red to pale orange-yellow. Below the chromatophores is a thin layer of special bodies called iridocytes that break up white light to give green and blue. Thus the octopus has a vast repertoire of colour patterns that it can adopt, to make itself inconspicuous on any sort of marine background.

This feature was published on December 21, 2003